1. Field of the Invention
The present invention relates to optical systems for performing geometrical measurements, and more particularly it concerns a method of and a device for performing geometrical measurements on objects, even while same are moving.
2. Description of Related Art
An important problem arising in production of objects such as metal wires, textile fibres, optical fibres, or extruded or turned materials, is the continuous and on-line measurement of their geometrical sizes, such as their diameter or thickness (or generally the distance between two edges), or of the position of an edge relative to a reference.
A general requirement is to measure even very small sizes (e.g. of the order of the hundredths of millimetre) with high precision and without interfering with the manufacturing process.
The measurement techniques best suited for the attainment of those objectives are based upon an optical processing of a beam illuminating the object to be measured. An image containing the requested dimensional information is formed on a detector and the information is then extracted through a suitable electronic processing. Those techniques allow performing an on-line measurement without direct contact with the object to be measured, thus without stopping or slowing down the manufacturing process.
Among those techniques, those based upon the analysis of the light diffracted by the object when the latter is illuminated by a well-collimated laser beam are particularly interesting. Examples are disclosed in the paper “Absolute diameter measurement”, by J. Kiss, Wire & Cable Technology International, March 1999, pages 193 to 194, and in WO-A 8904946. In such examples, the diffracted light is collected by a charge-coupled device (CCD) detector, of which the size is equal to the entire measurement range. Such apparatuses do not include moving mechanical parts, contrary to commercially available instruments based upon the scanning of a laser beam by means of rotary mirrors. Therefore such apparatuses are more reliable, even if signal processing is still rather complex.
Another technique is disclosed for instance by F. Docchio et al. in the paper “On-Line Dimensional Analysis of Surfaces Using Optical Filtering and Elaboration Techniques in the Fourier Plane”, IEEE Transactions on Instrumentation and Measurement, Vol. 38, No. 3, June 1989, pages 811 to 814. According to such technique, the diffracted beam is submitted to a spatial filtering carried out according to the Fourier optics techniques. For instance, for measuring the diameter of a wire, the filtering is carried out by means of a high-pass filter comprising a circular opaque member of suitable diameter placed on the optical axis of the system. The filtering enhances the image contour and provides on the detector an intensity distribution comprising a very narrow peak in correspondence with the object edges. The measurement of the object is thus made easier.
Otherwise stated, the spatial filtering allows getting a very “clean” electrical signal, from which the requested measure can be obtained. Yet such a result, even if analytically valid, can be obtained in practice only by using an optical system very close to an ideal system, that is a system in which:                the laser source is perfectly collimated and monochromatic;        the object under measurement is exactly placed in the focal plane of a lens of the optical processing system;        the lenses are thin lenses, without aberrations;        the lens apertures are very great if compared to the size to be measured.        
The practical experience shows how difficult actually is to meet such conditions during an industrial manufacturing process. Such conditions can be reproduced at most in a laboratory, by using complex and therefore expensive instrumentation and by performing the measurements on carefully positioned objects, preferably on stationary objects.
If one attempts to build an apparatus that does not meet even only one of the ideal requirements, there is experimentally observed that a narrow intensity peak is no longer created, but a loss of definition of the peak is observed (this is the usual experience of an out-of-focus image, where the figure contours are no longer sharp), together with an increase of the background noise due to high spatial frequency components which become quantitatively important. The determination of the exact points corresponding to the object edges would therefore be affected by a non negligible uncertainty, which is not acceptable if a sub-micron precision is required.